Secretary-General Antonio Guterres warned Monday that the world is facing "a direct existential threat" and must rapidly shift from dependence on fossil fuels by 2020 to prevent "runaway climate change."

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The UN chief called the crisis urgent and decried the lack of global leadership to address global warming.

Guterres said scientists have been warning about global warming for decades, but "far too many leaders have refused to listen - far too few have acted with the vision the science demands."

When some 190 nations signed the 2015 Paris agreement on climate change they agreed to limit the global temperature increase by 2100 to less than 2 degrees Celsius (3.6 degrees Fahrenheit) and as close as possible to 1.5 degrees.

"These targets were the bare minimum to avoid the worst impacts of climate change," Guterres said. "But scientists tell us that we are far off track."

"According to a UN study, the commitments made so far by parties to the Paris agreement represent just one-third of what is needed," the secretary-general said.

Guterres said the mountain that needs to be climbed is very high - but not insurmountable.

"We need to rapidly shift away from our dependence on fossil fuels," he said. "We need to replace them with clean energy from water, wind and sun. We must halt deforestation, restore degraded forests and change the way we farm."

He appealed for leadership - "from politicians and leaders, from business and scientists, and from the public everywhere" - to break what he called the current "paralysis" and act now.

"If we do not change course by 2020, we risk missing the point where we can avoid runaway climate change, with disastrous consequences for people and all the natural systems that sustain us," Guterres warned.

The alternative to moving to green energy, he said, "is a dark and dangerous future."

Guterres said that when he addresses world leaders at their annual General Assembly gathering in two weeks, he will tell them "that climate change is the great challenge of our time" and what is missing is leadership and a sense of urgency to respond.

He said an international meeting in Bangkok that ended Sunday made some progress on negotiations to help reach an agreement in December in Poland on guidelines for implementing the 2015 Paris accord - "but far from enough."

"Nothing less than our future and the fate of humankind depends on how we rise to the climate challenge," Guterres said. "Keeping our planet's warming to well below 2 degrees (Celsius) is essential for global prosperity, people's well-being and the security of nations."

He said that is why he will convoke a climate summit for world leaders in September 2019 "to bring climate action to the top of the international agenda."

Guterres said technology is on the side of those seeking to tackle climate change.

He cited the rising use of renewable energy, saying "today, it is competitive with - and even cheaper - than coal and oil, especially if one factors in the cost of pollution." And he singled out innovative programs in China, Sweden, Morocco, Scotland and Thailand.

Guterres also pointed to other signs of hope including oil-rich Saudi Arabia investing heavily in renewable energy and oil-rich Norway's sovereign wealth fund - the largest in the world - moving away from investments in coal as well as in palm and pulp paper companies because of the forests they destroy.